Unless I'm mistaken, when you run games in full screen exclusive mode, Fire Fox being open doesn't use any VRAM. It will of course still use some system memory. I don't really get what the issue is. Fire Fox uses about 800MB Vram for me at 4K.

Unless I'm mistaken, when you run games in full screen exclusive mode, Fire Fox being open doesn't use any VRAM. It will of course still use some system memory. I don't really get what the issue is. Fire Fox uses about 800MB Vram for me at 4K.

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Background app VRAM usage will be unloaded if a foreground app will require more VRAM than is currently available. This is a non-issue completely, in exclusive or non-exclusive fullscreen. And the OP has GTX1080 with 8GBs. It's kinda pointless to even worry about this on an 8GB card. There are no games which will consume 8GBs of VRAM even in 4K with MSAA.

I don't know if this supposed VRAM unloading would be new to Windows 8/10, but on Windows 7 x64, it can be a real issue. Case in point, on my 6 GB 1060 the new Wolfenstein II uses about 5.3-5.4 GB VRAM@1080p, everything maxed and image streaming on Ultra (second highest setting, I assume the highest is meant for 8 GB cards). Firefox nowadays is a real VRAM hog, so it fills 700 MB of VRAM in a breeze, maxing out VRAM entirely. So keeping Firefox running in the background means severe stutter in this game, on settings that otherwise run perfectly fine. Dropping this image streaming setting by one notch gets rid of the stuttering again, as it reduces the game's VRAM usage.

Disabling HW acceleration in Firefox is a solution, but this does introduce tearing in videos.

Background app VRAM usage will be unloaded if a foreground app will require more VRAM than is currently available. This is a non-issue completely, in exclusive or non-exclusive fullscreen. And the OP has GTX1080 with 8GBs. It's kinda pointless to even worry about this on an 8GB card. There are no games which will consume 8GBs of VRAM even in 4K with MSAA.

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I often leave the browser window (currently Firefox because Chrome got worse and worse through the last few updates for me) on my secondary display while playing a game on the high-res main display.
Can the driver still "swap" the VRAM content (at least from background tabs inside the browser)?

I didn't observe such thing. I see the extra overall VRAM usage (brought in by FF) persisting while gaming (if FF left open) and it went pretty close to 8Gb at some point (with the game running in 4k).
Although, I am aware that this VRAM utilization (as shown by Windows or AfterBurner) should not be treated as an exact measurement but rather informational only, so the real utilization might be more or less...

By the way, having ~10 tabs open seems to increase my VRAM usage by ~2 Gb (yes, you read that correctly!). I noticed this and wasn't happy but my overall user experience is still better than it was with Chrome lately.